The present invention relates to a receiver for a sounder permitting the detection and measurement of phenomena connected with the earth's environment. The receiver is applicable to sounders, which make it possible to study phenomena linked with the earth's environment and particularly sounders used for studying the ionosphere and especially the study of short-term, low-amplitude modifications of the electron density gradient of the ionosphere. This receiver can also be used in sounders making it possible to study other phenomena, such as e.g. the study or monitoring of sea swells.
It is known that the ionosphere is the region at altitudes above 70 km in which, under the influence of solar radiation, a fraction of the atoms and molecules forming the earth's atmosphere is subdivided into positive ions and negative electrons. At hig-h altitudes, the rarefaction of the atmosphere is such that the electrons have a low density. At low altitudes, the atmosphere is much more dense. However, the energy of the solar radiation has been consumed at high altitude for the ionization of the molecules and the residual energy of this radiation is no longer sufficient to produce a large ionization. As a result, at low altitude, the ionosphere forms a shield protecting the surface of the earth against certain dangerous ultraviolet rays. The greatest ionization takes place at an altitude of about 200 km and reaches a few hundred thousand electrons/cm.sup.3 (10.sup.5 to 10.sup.6 /cm.sup.3). Thus, the most characteristic quantity of the ionosphere is the electron density or ionization density. Knowledge of this density at different altitudes makes it possible to detect the disturbances undergone by the ionosphere as a result of events taking place on earth (e.g. earthquakes). The knowledge of the electron density of the ionosphere also makes it possible to study sudden ionospheric disturbances produced by solar flares. These disturbances lead to a sudden increase in electromagnetic radiation emitted towards the earth and particularly in the X-ray range. These solar eruptions can also give rise to significant fluctuations in the earth's magnetic field, called magnetic storms. These storms give rise to an increase in ionization in auroral regions and their effects can be observed down to low altitude regions. Finally, the ionosphere is subject to large day and night oscillations involving tides and gravity waves. The tides are excited by the daytime heating of atmospheric ozone. Gravity waves are excited by meteorological fronts or by auroral disturbances. The ionosphere is also subject to slow variations linked with the seasons or the solar cycle.
Several types of sounders are known, which make it possible to detect phenomena linked with the earth and its environment, particularly phenomena relating to the ionosphere. One of these known sounders is described in U.S. Pat. No. 4,356,487 in the name of the present Applicant and which uses the radar principle. This sounder comprises a channel for emitting electromagnetic signals in the direction of the ionosphere and a receiver making it possible to intercept the echoes of these signals on different layers of the ionosphere. The electromagnetic waves emitted are general pulse-modulated waves. These pulses are emitted in the direction of the ionosphere, where the phenomenon to be detected and the resulting echoes are intercepted by the receiver, which detects these echoes as a function of their amplitude, their frequency and their frequency shift. Knowing the pulse emission times and the reception times of the echoes thereof, together with their frequency, this detection makes it possible to determine the distance separating this phenomenon from the emitter. In the case of ionospheric sounding, an ionospheric layer only reflects the electromagnetic waves, whose frequency is below the critical frequency linked with the maximum electron density of said layer. The waves emitted at a predetermined frequency and below the critical frequency, supply echoes at a given time with respect to the emission time, so that it is possible to establish the altitude of the ionospheric region, whose electron density is that necessary for the reflection of the electromagnetic sounding wave. In order to give more information on the evolution of the phenomena studied and particularly the ionized regions, the sounder described in the aforementioned U.S. Patent also makes it possible to detect small electron density variations as a result of the use of the Doppler radar method. Such ionospheric electron density variations as a function of time lead to frequency shifts compared with the sounding frequency of the echoes resulting from the waves emitted in the direction of the atmosphere and this is called the Doppler effect.
The sounder makes it possible to observe very small frequency shifts corresponding to variations in the phase path of the electromagnetic sounding waves of approximately lm/sec. Moreover, this sounder has a good spatial resolution determined by very small widths of the pulses emitted in the direction of the ionosphere.
At present, no receiver is known which permits, in a sounder emitting pulses on several sounding frequencies, to measure frequency shifts (Doppler effect) and also the envelope delays of the modulated signals emitted at different fixed frequencies. In general, these receivers are complicated and often the measurements are made impossible through the presence of distortions produced by the receiver and through the presence of high interference levels reaching the input thereof.
The object of the present invention is to obviate these disadvantages and in particular to provide a receiver for a sounder making it possible to detect and measure phenomena linked with the earth's environment. This receiver makes it possible to carry out measurements of Doppler frequencies and of envelope delays, without them being disturbed by the distortions produced by the receiver or by interference at its input. As will be shown in greater detail hereinafter, these objectives are achieved through the use of attenuators, very accurate filters which are insensitive to interfering signals and phase-cohtrolled oscillators.